High-precision wideband differential amplifiers have broad application in modern electronic equipment, particularly in measurement instruments such as oscilloscopes. One amplifier design which provides vastly improved performance, particularly in reduced nonlinearity and thermal distortion error, is the cascode feed-forward amplifier known in the industry as the "cascomp" amplifier taught by Patrick A. Quinn in U.S. Pat. No. 4,146,844, which is assigned to asignee of the present invention. One useful application of the cascomp amplifier is in the horizontal deflection amplifier circuit of a wide bandwidth oscilloscope. When using the oscilloscope to provide a horizontally-magnified display (magnified sweep mode), the horizontal amplifier is almost always overdriven, and the problem is that the magnifying amplifier must limit the current drive to the shunt feedback output stages to prevent the output amplifiers from going out of regulation due to saturation or cutoff of transistors in the loop, which would cause severe overdrive recovery problems.
Another application for a limiting cascomp is in the vertical chain either in the channel switch or the vertical output stage. In a so-called "add" mode, full dynamic range of both channels is necessary; however, this criterion causes subsequent stages to be greatly overdriven. The present invention limits the vertical signal after the adder stage to prevent transient-response and thermal-distortion recovery problems when overdrive conditions exist.
Thus it would be desirable to provide a cascomp amplifier which when overdriven provides a well-defined, well-behaved current limiting function.